Corrosion always develops at the anode, where current leaves the metal and enters the electrolyte, whilst a protective effect occurs at the cathode. Thus if the whole metal surface is made sufficiently cathodic, corrosion will not occur. This is the basic principle of Cathodic Protection. In marine structures, such corrosion cells may result from the use of dissimilar metals. Usually,however, localised anodic and cathodic areas arise on the surface of the same metal through differences in the metal itself, variations in protective films or changes in the electrolyte. ie: aeration, temperature and salinity. Corrosion may be prevented by removing one or more of these corrosive elements and for marine structures, the most practicable method is to apply a protective coating, thus introducing an electrical resistance between the metal and the electrolyte. Paint in various forms normally provides the first level of protection. However, even the most efficient coatings are subject to defects during application or service, with inevitable corrosion of the exposed metal.It is therefore generally accepted that cathodic protection, in conjunction with a high performance paint system provides the most effective and economic safeguard against corrosion on larger vessels or platforms.